Direct injection spark ignition (DISI) engines offer opportunities in reducing CO2 emissions and improving fuel economy. With potential for reduced pumping losses and improved thermal efficiency, the DISI engines, such as those capable of stratified charge combustion, can deliver improved fuel economy and with reduced emissions. Further, the additional degree of freedom in fuel delivery time in DISI engines can provide a mechanism for controlling exhaust temperature, such as during engine starting. On the other hand, DISI engines may generate higher feed-gas hydrocarbon emissions compared to conventional Port Fuel Injection (PFI) engines for both warmed-up steady state and cold start transient conditions.
In one particular approach using a DISI engine, a double injection may be combined with retarded spark timing to increase heat delivery during an engine start. In this way, more heat can be delivered to the catalyst and therefore shorten the catalyst light-off time and improve after treatment efficiency.
However, the inventors have recognized that the benefits from combining double injection with retarded spark timing are usually achieved at the expense of additional fuel economy and increased feed-gas hydrocarbon emissions. Thus, in some examples, even when using double injection with retarded spark, emission may still increase beyond a regulated value.
Thus, in one approach, a method for operating an engine having a four-stroke cycle including an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke, is provided. The method comprises: performing multiple ignitions during a single cycle, wherein a first ignition takes place during a first stroke in the cycle and a second ignition takes place during a second stroke in the cycle. Such operation may be performed during an engine cold start, for example, along with multiple injections (e.g., one injection during the first stroke and one during the second stroke), to increase exhaust gas heat while lowering emissions. In this way, it is possible to provide improved cold start up protocols that allow for the exploitation of the benefits of the DISI engines while minimizing emissions and optimizing system performance. Further, it is possible to improve engine robustness to low quality fuel during engine cold start performance.
In another approach, a method for optimizing port oxidation in an engine having a four-stroke cycle, including an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke is provided. The method comprises: performing a first pattern of multiple injections and at least one ignition during a first cycle; and performing a second pattern of multiple injections and multiple ignitions during a second cycle; where the first and second patterns are different.
In this way, appropriate ignition and injection modes can be selected for different operating conditions to improve operation overall across a plurality of different conditions. For example, in some cases, it may be desirable to burn as much fuel as possible in the cylinder, while in other cases it may be desirable to generate exothermic heat in the exhaust via excess reductants in the exhaust.